The present invention relates to a mooring system for tanker vessels that employs a vessel mounted turret in combination with a submerged chain table and a plurality of mooring lines to moor the vessel.
FPSOs are Floating Production, Storage and Offloading vessels which are employed to temporarily store oil received from offshore subsea and dry surface wells (or in the case of an FSO, pumped from offshore production facilities) until an oil tanker arrives to receive the oil from the FPSO. Typically, an FPSO is itself an oil tanker that has been modified to receive the oil directly from one or more subsea risers (flexible pipelines), and be moored in a fixed position near the offshore oil wellheads or production facilities. The mooring systems employed to hold the FPSOs in place must be specially designed for a number of reasons. In particular, the mooring system must not interfere with one or more subsea oil risers that are employed to transfer oil from the offshore production facility to the FPSO. Additionally, the mooring system must be designed to withstand adverse environmental conditions, including rough seas and strong winds, since offshore production facilities are typically located in areas subject to such conditions.
The foregoing requirements have resulted in the design of a number of different mooring arrangements for FPSOs. The most popular design is known as a single point turret mooring system in which only the bow (or stern) of the FPSO is attached to the mooring by means of a vessel mounted rotatable turret, thus allowing the FPSO to pivot or "weathervane" about a single axis of rotation in response to wind or water currents. Previous turret mooring systems have consisted of a number of mooring lines that are attached to the rotatable turret. The turret mooring is typically attached to the seabed by 5 to 20 mooring lines arranged in a radial or clustered pattern.
Due to the forces imparted on the lines by the vessel, the lines must be made to withstand high maximum loads, thus increasing their cost. In addition, these arrangements may require that a drive system be provided on the vessel for rotating the turret as the vessel weathervanes. This is necessary because frictional forces in the turret prevent the forces imparted by the mooring lines from turning the turret on their own. As a result, the mooring lines will get twisted and tangled if the drive system is not provided.